Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) technology is well known and widely used in the electronics industry. Performance enhancement between generations of devices is generally achieved by reducing the size of the device, resulting in an enhancement in device speed. This is generally referred to as device “scaling.” As MOSFETs are scaled to channel lengths below 100 nm, conventional MOSFETs suffer from several problems. In particular, interactions between the source and drain of the MOSFET degrade the ability of the gate of the same to control whether the device is on or off. This phenomenon is called the “short-channel effect”.
Silicon-on-insulator (SOI) MOSFETs are formed with an insulator (usually, but not limited to, silicon dioxide) below the device active region, unlike conventional “bulk” MOSFETs, which are formed directly on silicon substrates, and hence have silicon below the active region. SOI is advantageous since it reduces unwanted coupling between the source and the drain of the MOSFET through the region below the channel. This result is often achieved by ensuring that all the silicon in the MOSFET channel region can be depleted by the gate (called a fully depleted SOI MOSFET). As device size is scaled, however, this becomes increasingly difficult because the distance between the source and drain is reduced. The reduced distance increases interaction with the channel, reducing gate control and increasing short channel effects.
The double-gate MOSFET structure places a second gate in the device, such that there is a gate on either side of the channel. This allows gate control of the channel from both sides, reducing short channel effects. Additionally, when the device is turned on using both gates, two conduction (“inversion”) layers are formed, allowing for more current flow or higher drive current. An extension of the double-gate concept is the “surround-gate” or “wraparound-gate” concept, where the gate is placed such that it completely or almost-completely surrounds the channel, providing better gate control.
These double-gate MOSFETs are sometimes referred to as “FinFET” structures because of their shape. One method of forming FinFET structures is by forming channels and source and drain regions by etching SOI film. Resulting channel structure carries current along both sidewalls of the fin.
In silicon MOSFET devices, it has been shown that performance can be enhanced by enhancing the mobility of electrons and holes in, the channel region. One way to enhance mobility is by the use of strained materials, such as strained silicon. A material under appropriate stress can enhance electron and/or hole carrier mobility due to modulation or material's energy band structure by the strain.
Thus, there is a need for a method of forming a strained silicon finFET. Further, there is a need for enhanced channel mobility using double-gate MOSFETs and strained materials. Even further, there is a need for improved electron mobility in the channel region.